Color display device

ABSTRACT

The present invention provides a color display device in which each pixel or sub-pixel can display four high quality color states, More specifically, an electrophoretic fluid is provided which comprises four types of particles, dispersed in a solvent or solvent mixture. The fluid may further comprise substantially uncharged neutral buoyancy particles.

REFERENCE TO RELATED APPLICATIONS

This is a continuation of copending application Ser. No. 14/921,265,filed Oct. 23, 2015 (Publication No. 2016/0041448), which is acontinuation-in-part of application Ser. No. 14/279,173, filed May 15,2014 (now U.S. Pat. No. 9,170,468, issued Oct. 27, 2015); which claimsthe benefit of U.S. Provisional Application Ser. No. 61/824,887, filedMay 17, 2013; 61/893,831, filed Oct. 21, 2013; and 61/974,858, filedApr. 3, 2014. The contents of the above-identified applications areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to a color display device in whicheach pixel or sub-pixel can display four high quality color states, anelectrophoretic fluid for such an electrophoretic display and drivingmethods for such a display device.

BACKGROUND OF THE INVENTION

In order to achieve a color display, color filters are often used. Themost common approach is to add color filters on top of three black/whitesub-pixels of a pixellated display to display the red, green and bluecolors. When a red color is desired, the green and blue sub-pixels areturned to the black state so that the only color displayed is red. Whena blue color is desired, the green and red sub-pixels are turned to theblack state so that the only color displayed is blue. When a green coloris desired, the red and blue sub-pixels are turned to the black state sothat the only color displayed is green. When the black state is desired,all three-sub-pixels are turned to the black state. When the white stateis desired, the three sub-pixels are turned to red, green and blue,respectively, and as a result, a white state is seen by the viewer.

The disadvantage of such a technique is that the white state is fairlydim. To compensate it, a fourth sub-pixel may be added which can displayonly the black and white states. In this approach, the white level maybe increased, but the increase in whiteness is at the expense of thered, green or blue color level (where each of the sub-pixels is only onefourth of the area of a pixel). In other words, a brighter white colorcan be achieved, but it is achieved at the expense of the other colorswhich will become light and unsaturated. Even with this approach, thewhite level is normally still substantially less than half of that of ablack and white display, rendering it an unacceptable choice for displaydevices, such as e-readers or displays that need well readableblack-white brightness and contrast.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a display layercomprising an electrophoretic medium and having first and secondsurfaces on opposed sides thereof, the electrophoretic medium comprisinga first type of positive particles, a first type of negative particles,a second type of positive particles and a second type of negativeparticles, all dispersed in a solvent or solvent mixture, the four typeof particles having respectively optical characteristics differing fromone another, such that:

-   -   (a) application of an electric field which has the same polarity        as the first type of positive particles will cause the optical        characteristics of the first type of positive particles to be        displayed at the first surface; or    -   (b) application of an electric field which has the same polarity        as the first type of negative particles will cause the optical        characteristic of the first type of negative particles to be        displayed at the first surface; or    -   (c) once the optical characteristic of the first type of        positive particles is displayed at the first surface,        application of an electric field which has the same polarity as        the second type of negative particles, but is not strong enough        to overcome the attraction force between the first type of        positive particles and the first type of negative particles, but        sufficient to overcome the attraction forces between other        oppositely charged particles will cause the optical        characteristic of the second type of negative particles to be        displayed at the first surface; or    -   (d) once the optical characteristic of the first type of        negative particles is displayed at the first surface,        application of an electric field which has the same polarity as        the second type of positive particles, but is not strong enough        to overcome the attraction force between the first type of        positive particles and the first type of negative particles, but        sufficient to overcome the attraction forces between other        oppositely charged particles will cause the optical        characteristic of the second type of positive particles to be        displayed at the first surface.

In one embodiment, the first type of positive particles is blackparticles, the first type of negative particles is yellow particles, thesecond type of positive particles is the red particles and the secondtype of negative particles is the white particles.

In one embodiment, the charges of the first type of positive particlesand the first type of negative particles are higher than the second typeof positive particles and the second type of negative particles.

In one embodiment, the charges of the lower charged particles are lessthan 50% of the charges of the higher charged particles. In oneembodiment, the charges of the lower charged particles are 5% to 30% ofthe charges of the higher charged particles. In one embodiment, thecharges of the lower charged particles are less than 75% of the chargesof the higher charged particles. In one embodiment, the charges of thelower charged particles are 15% to 55% of the charges of the highercharged particles.

In one embodiment, the electrophoretic medium further comprisingsubstantially uncharged neutral buoyancy particles. In one embodiment,the neutral buoyancy particles are non-charged.

Another aspect of the present invention is directed to a driving methodfor an electrophoretic fluid comprising four types of charged pigmentparticles dispersed in a solvent or solvent mixture, wherein the fourtypes of charged pigment particles are high positive charged pigmentparticles, high negative charged pigment particles, low positive chargedpigment particles and low negative charged particles, which methodcomprises

-   -   (a) driving a pixel to the color state of one of the low charged        particles; followed by    -   (b) driving the pixel to the color state of high charged        particles, wherein the low charged particles and the high        charged particles carry opposite charge polarities.

In one embodiment, the method further comprises a shaking waveform.

In one embodiment of the driving method, the high positive chargedparticles are black. In another embodiment, the high negative chargedparticles are yellow. In a further embodiment, the low positive chargedparticles are red. In yet a further embodiment, the low negative chargedparticles are white.

A further aspect of the invention is directed to a driving method for anelectrophoretic fluid comprising four types of charged pigment particlesdispersed in a solvent or solvent mixture, wherein the four types ofcharged pigment particles are high positive charged pigment particles,high negative charged pigment particles, low positive charged pigmentparticles and low negative charged particles, which method comprises

-   -   (a) applying a shaking waveform;    -   (b) applying a high driving voltage having the same polarity as        one type of high charged pigment particles to drive to a color        state of the high charged pigment particles;    -   (c) applying a low driving voltage having the same polarity as        one type of low charged pigment particles to drive to a color        state of low charged pigment particles; and    -   (d) applying a high driving voltage having the same polarity as        the high charged pigment particles to drive to a color state of        the high charged pigment particles; wherein the high charged        pigment particles and the low charged pigment particles are        oppositely charged and the driving method is DC balanced.

In yet a further aspect of the invention is directed to anelectrophoretic fluid comprising four types of charged pigment particlesdispersed in a solvent or solvent mixture, wherein the four types ofcharged pigment particles are high positive charged pigment particles,high negative charged pigment particles, low positive charged pigmentparticles and low negative charged particles and the low chargedparticles have a charge intensity which is less than 75% of the chargeintensity of the high charged particles.

In one embodiment, the low positive charged particles have a chargeintensity which is less than 50% of the charge intensity of the highpositive charged particles and the low negative charged particles have acharge intensity which is less than 75% of the charge intensity of thehigh negative charged particles.

In one embodiment, the fluid further comprises substantially unchargedneutral buoyancy particles, which may be non-charged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a display layer which can display four different colorstates.

FIGS. 2-1 to 2-3 illustrate an example of the present invention.

FIG. 3 demonstrates display cells unaligned with pixel electrodes.

FIGS. 4A and 4B illustrate driving methods of the present invention.

FIG. 5 shows a shaking waveform which may be incorporated into drivingsequences.

FIGS. 6A and 6B show example waveforms for driving the display device ofthe present invention.

FIGS. 7A, 7B, 8A, 8B, 9A and 9B illustrate one aspect of the presentinvention wherein a pixel consists of two sub-pixels.

DETAILED DESCRIPTION OF THE INVENTION

The electrophoretic fluid of present invention comprises two pairs ofoppositely charged particles. The first pair consists of a first type ofpositive particles and a first type of negative particles and the secondpair consists of a second type of positive particles and a second typeof negative particles.

In the two pairs of oppositely charged particles, one pair carries acharge higher than the other pair. Therefore the four types of particlesmay also be referred to as high positive particles, high negativeparticles, low positive particles and low negative particles.

As an example shown in FIG. 1, the black particles (K) and yellowparticles (Y) are the first pair of oppositely charged particles, and inthis pair, the black particles are the high positive particles and theyellow particles are the high negative particles. The red particles (R)and the white particles (W) are the second pair of oppositely chargedparticles and in this pair, the red particles are the low positiveparticles and the white particles are the low negative particles.

In another example not shown, the black particles may be the highpositive particles; the yellow particles may be the low positiveparticles; the white particles may be the low negative particles and thered particles may be the high negative particles.

In addition, the color states of the four types of particles may beintentionally mixed. For example, because yellow pigment, by nature,often has a greenish tint and if a better yellow color state is desired,yellow particles along with red particles may be used where both typesof particles carry the same charge polarity and the yellow particles arehigher charged than the red particles. As a result, at the yellow state,there will be a small amount of the red particles mixed with the yellowparticles, which red particles neutralize the greenish tint leading to ayellow state of better color purity.

It is understood that the scope of the invention broadly encompassesparticles of any colors as long as the four types of particles havevisually distinguishable colors.

For the white particles, they may be formed from an inorganic pigment,such as TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ or the like.

For the black particles, they may be formed from CI pigment black 26 or28 or the like (e.g., manganese ferrite black spinel or copper chromiteblack spinel) or carbon black.

Particles of non-white and non-black colors are independently of a colorsuch as red, green, blue, magenta, cyan or yellow. The pigments forcolor particles may include, but are not limited to, CI pigment PR 254,PR122, PR149, PG36, PG58, PG7, PB15:3, PY83, PY138, PY150, PY155 orPY20. Those are commonly used organic pigments described in color indexhandbooks, “New Pigment Application Technology” (CMC Publishing Co, Ltd,1986) and “Printing Ink Technology” (CMC Publishing Co, Ltd, 1984).Specific examples include Clariant Hostaperm Red D3G 70-EDS, HostapermPink E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm BlueB2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, HostapermGreen GNX, BASF Irgazine red L 3630, Cinquasia Red L 4100 HD, andIrgazine Red L 3660 HD; Sun Chemical phthalocyanine blue, phthalocyaninegreen, diarylide yellow or diarylide AAOT yellow.

The color particles may also be inorganic pigments, such as red, green,blue and yellow. Examples may include, but are not limited to, CI (colorindex) pigment blue 36 or 28 (PB36 or PB28), CI pigment yellow 227 or53, CI pigment green 50 or 26, CI pigment red 102, and the like.

The pigment particles may be encapsulated or polymer-coated, prepared bymethods known in the art.

In addition to the colors, the four types of particles may have otherdistinct optical characteristics, such as optical transmission,reflectance, luminescence or, in the case of displays intended formachine reading, pseudo-color in the sense of a change in reflectance ofelectromagnetic wavelengths outside the visible range.

A display layer utilizing the display fluid of the present invention hastwo surfaces, a first surface (13) on the viewing side and a secondsurface (14) on the opposite side of the first surface (13). The displayfluid is sandwiched between the two surfaces. On the side of the firstsurface (13), there is a common electrode (11) which is a transparentelectrode layer (e.g., ITO), spreading over the entire top of thedisplay layer. On the side of the second surface (14), there is anelectrode layer (12) which comprises a plurality of pixel electrodes (12a).

The pixel electrodes are described in U.S. Pat. No. 7,046,228, thecontent of which is incorporated herein by reference in its entirety. Itis noted that while active matrix driving with a thin film transistor(TFT) backplane is mentioned for the layer of pixel electrodes, thescope of the present invention encompasses other types of electrodeaddressing as long as the electrodes serve the desired functions.

Each space between two dotted vertical lines in FIG. 1 denotes a pixelor a sub-pixel. As shown, each pixel or sub-pixel has a correspondingpixel electrode. An electric field is created for a pixel by thepotential difference between a voltage applied to the common electrodeand a voltage applied to the corresponding pixel electrode.

The percentages of the four types of particles in the fluid may vary.For example, in a fluid having black/yellow/red/white particles, theblack particle may take up 0.1% to 10%, preferably 0.5% to 5%, by volumeof the electrophoretic fluid; the yellow particle may take up 1% to 50%,preferably 5% to 15%, by volume of the fluid; and each type of the redand white particles may take up 2% to 20%, preferably 4% to 10%, byvolume of the fluid.

The solvent in which the four types of particles are dispersed is clearand colorless. It preferably has a low viscosity and a dielectricconstant in the range of about 2 to about 30, preferably about 2 toabout 15 for high particle mobility. Examples of suitable dielectricsolvent include hydrocarbons such as Isopar, decahydronaphthalene(DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, siliconfluids, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenatedsolvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride,chloropentafluorobenzene, dichlorononane or pentachlorobenzene, andperfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company,St. Paul Minn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoroethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

In one embodiment, the charge (i.e., intensity or magnitude) carried bythe “low charge” particles may be less than about 50%, preferably about5% to about 30%, of the charge carried by the “high charge” particles.In another embodiment, the charge carried by the “low charge” particlesmay be less than about 75%, or about 15% to about 55%, of the chargecarried by the “high charge” particles. In a further embodiment, thecomparison of the charge levels as indicated applies to two types ofparticles having the same charge polarity. Therefore, the chargedifferentials indicated above may be separately applied to the two pairsof high-low charged particles in the same medium. For example, themagnitude of the low positive particles may be less than 75% of themagnitude of the high positive particles and the magnitude of the lownegative particles may be less than 50% of the magnitude of the highnegative particles.

The magnitudes of the “high positive” particles and the “high negative”particles may be the same or different. Likewise, the magnitudes of the“low positive” particles and the “low negative” particles may be thesame or different.

It is also noted that in the same fluid, the two pairs of high-lowcharge particles may have different levels of charge differentials. Forexample, in one pair, the low positively charged particles may have acharge intensity which is 30% of the charge intensity of the highpositively charged particles and in another pair, the low negativelycharged particles may have a charge intensity which is 50% of the chargeintensity of the high negatively charged particles.

The charge intensity may be measured in terms of zeta potential. In oneembodiment, the zeta potential is determined by Colloidal DynamicsAcoustoSizer IIM with a CSPU-100 signal processing unit, ESA EN# Attnflow through cell (K:127). The instrument constants, such as density ofthe solvent used in the sample, dielectric constant of the solvent,speed of sound in the solvent, viscosity of the solvent, all of which atthe testing temperature (25° C.) are entered before testing. Pigmentsamples are dispersed in the solvent (which is usually a hydrocarbonfluid having less than 12 carbon atoms), and diluted to between 5-10% byweight. The sample also contains a charge control agent (Solsperse17000®, available from Lubrizol Corporation, a Berkshire Hathawaycompany; “Solsperse” is a Registered Trade Mark), with a weight ratio of1:10 of the charge control agent to the particles. The mass of thediluted sample is determined and the sample is then loaded into the flowthrough cell for determination of the zeta potential.

It is also noted that the four types of particles may have differentparticle sizes. For example, the smaller particles may have a size whichranges from about 50 nm to about 800 nm. The larger particles may have asize which is about 2 to about 50 times, and more preferably about 2 toabout 10 times, the sizes of the smaller particles.

The following is an example illustrating the present invention.

Example 1

This example is demonstrated in FIG. 2. The high positive particles areof the black color (K); the high negative particles are of a yellowcolor (Y); the low positive particles are of a red color (R); and thelow negative particles are of a white color (W).

In FIG. 2(a), when a high negative voltage potential difference (e.g.,−15V) is applied to a pixel for a time period of sufficient length, anelectric field is generated to cause the yellow particles (Y) to bepushed to the common electrode (21) side and the black particles (K)pulled to the pixel electrode (22 a) side. The red (R) and white (W)particles, because they carry weaker charges, move slower than thehigher charged black and yellow particles and as a result, they stay inthe middle of the pixel, with white particles above the red particles.In this case, a yellow color is seen at the viewing side.

In FIG. 2(b), when a high positive voltage potential difference (e.g.,+15V) is applied to the pixel for a time period of sufficient length,the positive voltage potential difference causes the particledistribution to be opposite of that shown in FIG. 2(a) and as a result,a black color is seen at the viewing side.

In FIG. 2(c), when a lower positive voltage potential difference (e.g.,+3V) is applied to the pixel of FIG. 2(a) (that is, driven from theyellow state) for a time period of sufficient length, an electric fieldis generated to cause the yellow particles (Y) to move towards the pixelelectrode (22 a) while the black particles (K) move towards the commonelectrode (21). However, when they meet in the middle of the pixel, theyslow down significantly and remain there because the electric fieldgenerated by the low driving voltage is not strong enough to overcomethe strong attraction between them. On the other hand, the electricfield generated by the low driving voltage is sufficient to separate theweaker charged white and red particles to cause the low positive redparticles (R) to move all the way to the common electrode (21) side(i.e., the viewing side) and the low negative white particles (W) tomove to the pixel electrode (22 a) side. As a result, a red color isseen. It is also noted that there are also attraction forces betweenweaker charged particles (e.g., R) with stronger charged particles ofopposite polarity (e.g., Y). However, these attraction forces are not asstrong as the attraction forces between two types of stronger chargedparticles (K and Y) and therefore they can be overcome by the electricfield generated by the low driving voltage. In other words, weakercharged particles and the stronger charged particles of oppositepolarity can be separated.

In FIG. 2(d), when a lower negative voltage potential difference (e.g.,−3V) is applied to the pixel of FIG. 2(b) (that is, driven from theblack state) for a time period of sufficient length, an electric fieldis generated which causes the black particles (K) to move towards thepixel electrode (22 a) while the yellow particles (Y) move towards thecommon electrode (21). When the black and yellow particles meet in themiddle of the pixel, they slow down significantly and remain therebecause the electric field generated by the low driving voltage is notsufficient to overcome the strong attraction between them. At the sametime, the electric field generated by the low driving voltage issufficient to separate the white and red particles to cause the lownegative white particles (W) to move all the way to the common electrodeside (i.e., the viewing side) and the low positive red particles (R)move to the pixel electrode side. As a result, a white color is seen. Itis also noted that there are also attraction forces between weakercharged particles (e.g., W) with stronger charged particles of oppositepolarity (e.g., K). However, these attraction forces are not as strongas the attraction forces between two types of stronger charged particles(K and Y) and therefore they can be overcome by the electric fieldgenerated by the low driving voltage. In other words, weaker chargedparticles and the stronger charged particles of opposite polarity can beseparated.

Although in this example, the black particles (K) is demonstrated tocarry a high positive charge, the yellow particles (Y) carry a highnegative charge, the red (R) particles carry a low positive charge andthe white particles (W) carry a low negative charge, in practice, theparticles carry a high positive charge, or a high negative charge, or alow positive charge or a low negative charge may be of any colors. Allof these variations are intended to be within the scope of thisapplication.

It is also noted that the lower voltage potential difference applied toreach the color states in FIGS. 2(c) and 2(d) may be about 5% to about50% of the full driving voltage potential difference required to drivethe pixel from the color state of high positive particles to the colorstate of the high negative particles, or vice versa.

The electrophoretic fluid as described above is filled in display cells.The display cells may be microcells as described in U.S. Pat. No.6,930,818, the content of which is incorporated herein by reference inits entirety. The display cells may also be other types ofmicro-containers, such as microcapsules, micro-channels or equivalents,regardless of their shapes or sizes. All of these are within the scopeof the present application.

As shown in FIG. 3, the display cells (30), in the present invention,and the pixel electrodes (32 a) do not have to be aligned.

Ideally when a high positive driving voltage (e.g. +15V) is applied asshown in FIG. 2(b), the electric field generated would cause the highpositive black particles to move towards the common electrode side(i.e., the viewing side) and the high negative yellow particles and thelow negative white particles to move towards the non-viewing side. Thelow positive red particles would move towards the viewing side. Butsince the red particles carry a lower charge compared to the blackparticles, they move slower and as a result, the black color is seen atthe viewing side. However, in practice, the black state achieved mayhave a reddish tint. This could be caused by some of the red particlesbecoming mixed with the black particles at the viewing side.

The present invention also provides driving methods which can resolvethe unsatisfactory color issue. In one of the driving methods, a pixelis first driven towards the color state of one of the low chargedparticles before being driven towards the color state of high chargedparticles, wherein the low charged particles and the high chargedparticles carry opposite charge polarities.

For example, a pixel may be driven towards the black color state,according to the following steps:

-   -   a) driving first to the color state of the white particles (low        negative charged) by applying a low negative driving voltage;        and    -   b) driving towards the color state of the black particles (high        positive charged) by applying a high positive driving voltage.

This driving sequence is illustrated in FIG. 4A.

In step (a), once at the white state (e.g., FIG. 2(d)), the two types of“high charged” particles, black and yellow, will attract to each otherto cause them to stay in the middle of the pixel and the low positivecharged red pigment particles would move to be near or at the pixelelectrode.

In step (b), the white and yellow particles are pushed to the pixelelectrode side, and the low positive charged red particles are much lesslikely to show up at the viewing side. This sequence will result in abetter quality of the black state.

In this driving method, a white color state is driven directly towardsthe black state without going through the red or yellow color state. Ithas also been found that higher the quality of the white state in step(a) will lead to a higher quality of the black state in step (b). The“higher quality of the white state” simply means a high L* value and lowa* and b* values in the L*a*b* color system, for the white state.

A similar driving method may be applied to driving a pixel to the yellowstate. The method will have the following steps:

-   -   a) driving first to the color state of the red particles (low        positive charged) by applying a low positive driving voltage;        and    -   b) driving towards the color state of the yellow particles (high        negative charged) by applying a high negative driving voltage.

This driving sequence is shown in FIG. 4B.

In this driving method, a red color state is driven directly towards theyellow state without going through the white or black color state. Ithas also been found that higher the quality of the red state in step (a)will lead to a higher quality of the yellow state in step (b). The“higher quality of the red state” simply means a high a* value in theL*a*b* color system, for the red state.

In addition, to ensure both color brightness and color purity, a shakingwaveform, prior to driving from one color state to another color state,may be used. The shaking waveform consists of repeating a pair ofopposite driving pulses for many cycles. For example, the shakingwaveform may consist of a +15V pulse for 20 msec and a −15V pulse for 20msec and such a pair of pulses is repeated for 50 times. The total timeof such a shaking waveform would be 2000 msec (see FIG. 5).

In practice, there may be at least 10 repetitions (i.e., ten pairs ofpositive and negative pulses).

The shaking waveform may be applied regardless of the optical state(e.g., black, white, red or yellow) prior to a driving voltage isapplied. After the shaking waveform is applied, the optical state wouldnot be a pure white, pure black, pure yellow or pure red. Instead, thecolor state would be from a mixture of the four types of pigmentparticles.

Each of the driving pulses in the shaking waveform is applied for notexceeding 50% (or not exceeding 30%, 10% or 5%) of the driving timerequired from the full black state to the full yellow state or viceversa in the example. For example, if it takes 300 msec to drive adisplay device from a full black state to a full yellow state or viceversa, the shaking waveform may consist of positive and negative pulses,each applied for not more than 150 msec. In practice, it is preferredthat the pulses are shorter.

In one embodiment, a shaking waveform may be optionally applied prior tothe driving sequence of FIG. 4A or FIG. 4B.

For example, a pixel may be:

(i) applied a shaking waveform;

(ii) driven to black (i.e., the first-time black state);

(iii) driven to white; and then

(iv) driven to black (i.e., the second-time black state).

In this sequence, step (ii) may be carried out according to FIG. 2(b);step (iii) may be carried out according to FIG. 2(d); and step (iv) maybe carried out according to FIG. 4A.

An example waveform for this driving sequence is shown in FIG. 6A. Inany of the driving sequences of the present invention, the waveforms arepreferably DC balanced, that is, the average voltage applied across thedisplay is substantially zero when integrated over a time period. InFIG. 6A, in the initial step as shown, a high negative driving voltageis applied to ensure DC balance of the entire waveform.

Similarly, a pixel may be:

(i) applied a shaking waveform;

(ii) driven to yellow (i.e., the first-time yellow state);

(iii) driven to red; and then

(iv) driven to yellow (i.e., the second-time yellow state).

In this sequence, step (ii) may be carried out according to FIG. 2(a);step (iii) may be carried out according to FIG. 2(c); and step (iv) maybe carried out according to FIG. 4B.

An example waveform for this driving sequence is shown in FIG. 6B, whichis also “DC balanced”.

In practice, the quality of the first-time color state (black or yellow)is usually inferior compared with the second-time color state (black oryellow).

In a further aspect of the present invention, each pixel may consist oftwo sub-pixels. Each sub-pixel can display four different color states,from a display fluid comprising four types of particles dispersed in asolvent or solvent mixture, and the four types of particles are blackparticles and three types of non-black particles of different colors. Inaddition, as described above, the four types of particles are highpositive particles, high negative particles, low positive particles andlow negative particles, respectively. Furthermore, among the six typesof non-black particles between the two sub-pixels, there are at leastfour different colors.

The four different color states of each sub-pixel are displayed,according to the driving schemes described above.

In one embodiment, the six types of non-black particles between the twosub-pixels have six different colors. For example, the six colors may bered, green, blue, cyan, magenta and yellow, respectively. An example ofthis embodiment is illustrated in FIGS. 7A and 7B.

As shown in FIG. 7A, there are two sub-pixels (a) and (b) forming apixel. In sub-pixel (a), the four types of particles are black, red (R),green (G) and blue (B), and therefore sub-pixel (a) can display fourdifferent color states, black, R, G and B. In sub-pixel (b), the fourtypes of particles are black, cyan (C), magenta (M) and yellow (Y), andtherefore sub-pixel (b) can display four different color states black,C, M and Y.

As shown in FIG. 7B, the pixel consisting of sub-pixel (a) and sub-pixel(b) may display a black color when both sub-pixels display the blackcolor. The pixel may display a red, green, blue, cyan, magenta or yellowcolor when one sub-pixel displays the color of red, green, blue, cyan,magenta or yellow, and the other sub-pixel displays the black color. Thepixel may display a white color, when sub-pixel (a) displays the redcolor and the sub-pixel (b) displays the cyan color, or when sub-pixel(a) displays the green color and the sub-pixel (b) displays the magentacolor, or sub-pixel (a) displays the blue color and the sub-pixel (b)displays the yellow color.

In another embodiment, there are five different colors among the sixtypes of non-black particles between the two sub-pixels. In other words,two types of the particles, one from each sub-pixel, have the samenon-black color, preferably white. The remaining four types of particleshave four different non-black colors. An example of this embodiment isillustrated in FIGS. 8A and 8B.

As shown in FIG. 8A, there are two sub-pixels (a) and (b) forming apixel. In sub-pixel (a), the four types of particles are black, white,red (R) and blue (B), and therefore sub-pixel (a) can display fourdifferent color states, black, white, R and B. In sub-pixel (b), thefour types of particles are black, white, green (G) and magenta (M), andtherefore sub-pixel (b) can display four different color states black,white, G and M. In this example, among the six non-black particles,there are five different colors, white, red, blue, green and magenta.

As shown in FIG. 8B, the pixel consisting of sub-pixel (a) and sub-pixel(b) may display a black color when both sub-pixels display the blackcolor. The pixel may display a white color when both sub-pixels displaythe white color. The pixel may display a red, blue, green or magentacolor when one sub-pixel displays the color of red, blue, green ormagenta, and the other sub-pixel displays the black color. The pixel maydisplay a cyan color when sub-pixel (a) displays the blue color and thesub-pixel (b) displays the green color. The pixel may display a yellowcolor when sub-pixel (a) displays the red color and sub-pixel (b)displays the green color.

In a further embodiment, there are four different colors among the sixnon-black particles between the two sub-pixels. In other words, one setof two types of the particles, one from each sub-pixel, have the samenon-black color, preferably white and another set of two types ofparticles, one from each sub-pixel have the same color. The remainingtwo types of particles have two different non-black colors. An exampleof this embodiment is illustrated in FIGS. 9A and 9B.

As shown in FIG. 9A, there are two sub-pixels (a) and (b) forming apixel. In sub-pixel (a), the four types of particles are black, white,red (R) and green (G), and therefore sub-pixel (a) can display fourdifferent color states, black, white, R and G In sub-pixel (b), the fourtypes of particles are black, white, red (R) and blue (B), and thereforesub-pixel (b) can display four different color states black, white, Rand B. In this example, among the six non-black particles, there arefour different colors, white, red, green and blue.

As shown in FIG. 9B, the pixel may display the black or white stateswhile both sub-pixels display the black or white states. The pixel maydisplay a green or blue color state while one sub-pixel displays thegreen or blue color state and the other sub-pixel displays the blackstate. The pixel may display a red color state while both sub-pixelsdisplay the red color states or one sub-pixel displays the red colorstate and the other sub-pixel displays the black color state. The pixelmay display a cyan color while one sub-pixel (a) displays a green colorand sub-pixel (b) displays a blue color. The pixel may display a magentacolor while sub-pixel (a) displays a red color and sub-pixel (b)displays a blue color. The pixel may display a yellow color whilesub-pixel (a) displays a green color and sub-pixel (b) displays a redcolor.

The advantage of this aspect of the invention is that multiple colorstates may be achieved with only four types of charged particles in adisplay medium, and therefore the display device has a higher switchingspeed. In addition, if there is a color state (e.g., red) the colorintensity of which is weaker than other colors, the scheme of FIG. 9 maybe followed, in which the red color may be enhanced while bothsub-pixels display the red color.

In a further aspect of the present invention, the fluid comprising fourtypes of charged pigment particles may further comprise substantiallyuncharged neutral buoyancy particles.

The term “substantially uncharged” refers to the particles which areeither uncharged or carry a charge magnitude which is less than 5% ofthe average charge magnitude carried by the higher charged particles. Inone embodiment, the neutral buoyancy particles are non-charged.

The term “neutral buoyancy” refers to particles which do not rise orfall with gravity. In other words, the particles would float in thefluid between the two electrode plates. In one embodiment, the densityof the neutral buoyancy particles may be the same as the density of thesolvent or solvent mixture in which they are dispersed.

The concentration of the substantially uncharged neutral buoyancyparticles in the display fluid is preferably in the range of about 0.1to about 10% by volume, more preferably in the range of about 0.1 toabout 5% by volume.

The term “about” refers to a range which is ±10% of the indicated value.

The substantially uncharged neutral buoyancy particles may be formedfrom a polymeric material. The polymeric material may be a copolymer ora homopolymer.

Examples of the polymeric material for the substantially unchargedneutral buoyancy particles may include, but are not limited to,polyacrylate, polymethacrylate, polystyrene, polyaniline, polypyrrole,polyphenol and polysiloxane. Specific examples of the polymeric materialmay include, but are not limited to, poly(pentabromophenylmethacrylate), poly(2-vinylnapthalene), poly(naphthyl methacrylate),poly(alpha-methyl styrene), poly(N-benzyl methacrylamide) andpoly(benzyl methacrylate).

More preferably, the substantially uncharged neutral buoyancy particlesare formed from a polymer which is not soluble in the solvent of thedisplay fluid, and also has a high refractive index. In one embodiment,the refractive index of the substantially uncharged neutral buoyancyparticles is different from that of the solvent or solvent mixture inwhich the particles are dispersed. However, typically the refractiveindex of the substantially uncharged neutral buoyancy particles ishigher than that of the solvent or solvent mixture. In some cases, therefractive index of the substantially uncharged neutral buoyancyparticles may be above 1.45.

In one embodiment, the materials for the substantially uncharged neutralbuoyancy particles may comprise an aromatic moiety.

The substantially uncharged neutral buoyancy particles may be preparedfrom monomers through polymerization techniques, such as suspensionpolymerization, dispersion polymerization, seed polymerization,soap-free polymerization, emulsion polymerization or physical method,including inverse emulsification-evaporation process. The monomers arepolymerized in the presence of a dispersant. The presence of thedispersant allows the polymer particles to be formed in a desired sizerange and the dispersant may also form a layer physically or chemicallybonded to the surface of the polymer particles to prevent the particlesfrom agglomeration.

The dispersant preferably has a long chain (of at least eight atoms),which may stabilize the polymer particles in a hydrocarbon solvent. Suchdispersants may be an acrylate-terminated or vinyl-terminatedmacromolecule, which are suitable because the acrylate or vinyl groupcan co-polymerize with the monomer in the reaction medium.

One specific example of the dispersant is acrylate terminatedpolysiloxane (Gelest, MCR-M17, MCR-M22),

Another type of suitable dispersants is polyethylene macromonomers, asshown below:CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂

The backbone of the macromonomer may be a polyethylene chain and theinteger “n” may be 30-200. The synthesis of this type of macromonomersmay be found in Seigou Kawaguchi et al, Designed Monomers and Polymers,2000, 3, 263.

If the fluid system is fluorinated, the dispersants are then preferablyalso fluorinated.

Alternatively, the substantially uncharged neutral buoyancy particlesmay also be formed from a core particle coated with a polymeric shelland the shell may be formed, for example, from any of the polymericmaterial identified above.

The core particle may be of an inorganic pigment such as TiO₂, ZrO₂,ZnO, Al₂O₃, CI pigment black 26 or 28 or the like (e.g., manganeseferrite black spinel or copper chromite black spinel), or an organicpigment such as phthalocyanine blue, phthalocyanine green, diarylideyellow, diarylide AAOT yellow, and quinacridone, azo, rhodamine,perylene pigment series from Sun Chemical, Hansa yellow G particles fromKanto Chemical, and Carbon Lampblack from Fisher, or the like.

In the case of core-shell substantially uncharged neutral buoyancyparticles, they may be formed by a microencapsulation method, such ascoacervation, interfacial polycondensation, interfacial cross-linking,in-suit polymerization or matrix polymerization.

The size of the substantially uncharged neutral buoyancy particles ispreferably in the range of about 100 nanometers to about 5 microns.

In one embodiment of this aspect of the present invention, thesubstantially uncharged neutral buoyancy particles added to the fluidmay have a color substantially the same visually to the color of one ofthe four types of charged particles. For example, in a display fluid,there may be charged black, yellow, red and white particles andsubstantially uncharged neutral buoyancy particles, and in this case,the substantially uncharged neutral buoyancy particles may be black,yellow, red or white.

In another embodiment, the substantially uncharged neutral buoyancyparticles may have a color substantially different from the color ofeither one of the four types of charged particles.

The presence of the substantially uncharged neutral buoyancy particlesin the fluid increases reflection of incident light, thus also improvingthe contrast ratio, especially if they are formed from a reflectivematerial.

The image stability may also be improved by the addition of thesubstantially uncharged neutral buoyancy particles in the four particlefluid system. The substantially uncharged neutral buoyancy particles canfill in the gaps resulted from the charged particles being over packedon the surface of an electrode under an electrical field, thuspreventing the charged particles from settling due to the gravitationalforce.

In addition, if the substantially uncharged neutral buoyancy particlesare white, they may enhance the reflectivity of the display. If they areblack, they may enhance the black color displayed.

In any case, the substantially uncharged neutral buoyancy particles donot affect the driving behavior of the four types of charged particlesin the fluid.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particularsituation, materials, compositions, processes, process step or steps, tothe objective, spirit and scope of the present invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

The invention claimed is:
 1. A display layer having a first, viewingsurface and a second surface on the opposed side of the display layerfrom the first surface, the display layer further comprising anelectrophoretic medium sandwiched between two electrodes and comprisinga solvent or solvent mixture and high positively charged particles, highnegatively charged particles, low positively charged particles, and lownegatively charged particles all dispersed in the solvent or solventmixture, the four types of particles having respectively opticalcharacteristics differing from one another, such that: (a) applicationbetween the two electrodes of a high potential difference having apolarity driving the high positively charged particles towards theviewing surface will cause the display layer to display the opticalcharacteristic of the high positively charged particles at the viewingsurface; (b) application between the two electrodes of a high potentialdifference having a polarity driving the high negatively chargedparticles towards the viewing surface will cause the display layer todisplay the optical characteristic of the high negatively chargedparticles at the viewing surface; (c) once the optical characteristic ofthe high positively charged particles is displayed at the viewingsurface, application between the two electrodes of a low potentialdifference having a polarity driving the low negatively chargedparticles towards the viewing surface will cause the display layer todisplay the optical characteristic of the low negatively chargedparticles at the viewing surface; and (d) once the opticalcharacteristic of the high negatively charged particles is displayed atthe viewing surface, application between the two electrodes of a lowpotential difference having a polarity driving the low positivelycharged particles towards the viewing surface will cause the displaylayer to display the optical characteristic of the low positivelycharged particles at the viewing surface.
 2. A display layer accordingto claim 1, wherein the charge on the low charged particles is less than50% of the charge on the high charged particles.
 3. A display layeraccording to claim 1, wherein the charge on the low charged particles isless than 75% of the charge on the high charged particles.
 4. A displaylayer according to claim 1, wherein the low potential difference is from5% to 50% of the high potential difference.
 5. A display layer accordingto claim 1, wherein the electrophoretic medium further comprisessubstantially uncharged neutral buoyancy particles.
 6. Anelectrophoretic medium having a first, viewing surface and a secondsurface on the opposed side of the electrophoretic medium from the firstsurface, the electrophoretic medium being sandwiched between twoelectrodes and comprising a solvent or solvent mixture and highpositively charged particles, high negatively charged particles, lowpositively charged particles, and low negatively charged particles alldispersed in the solvent or solvent mixture, the four types of particleshaving respectively optical characteristics differing from one another,such that: (a) application between the two electrodes of a highpotential difference having a polarity driving the high positivelycharged particles towards the viewing surface will cause the displaylayer to display the optical characteristic of the high positivelycharged particles at the viewing surface; (b) application between thetwo electrodes of a high potential difference having a polarity drivingthe high negatively charged particles towards the viewing surface willcause the display layer to display the optical characteristic of thehigh negatively charged particles at the viewing surface; (c) once theoptical characteristic of the high positively charged particles isdisplayed at the viewing surface, application between the two electrodesof a low potential difference having a polarity driving the lownegatively charged particles towards the viewing surface will cause thedisplay layer to display the optical characteristic of the lownegatively charged particles at the viewing surface; and (d) once theoptical characteristic of the high negatively charged particles isdisplayed at the viewing surface, application between the two electrodesof a low potential difference having a polarity driving the lowpositively charged particles towards the viewing surface will cause thedisplay layer to display the optical characteristic of the lowpositively charged particles at the viewing surface.
 7. Anelectrophoretic medium according to claim 6, wherein the charge on thelow charged particles is less than 50% of the charge on the high chargedparticles.
 8. An electrophoretic medium according to claim 6, whereinthe charge on the low charged particles is less than 75% of the chargeon the high charged particles.
 9. An electrophoretic medium according toclaim 6, wherein the low potential difference is from 5% to 50% of thehigh potential difference.
 10. An electrophoretic medium according toclaim 6 further comprising substantially uncharged neutral buoyancyparticles.
 11. A method of driving a display comprising anelectrophoretic medium and having a first, viewing surface and a secondsurface on the opposed side of the electrophoretic medium from the firstsurface, the electrophoretic medium comprising a solvent or solventmixture and high positively charged particles, high negatively chargedparticles, low positively charged particles, and low negatively chargedparticles all dispersed in the solvent or solvent mixture, the fourtypes of particles having respectively optical characteristics differingfrom one another, which method comprises, in any order: (a) applyingacross the electrophoretic medium a high electric field having apolarity driving the high positively charged particles towards theviewing surface to cause the display layer to display the opticalcharacteristic of the high positively charged particles at the viewingsurface; (b) applying across the electrophoretic medium a high electricfield having a polarity driving the high negatively charged particlestowards the viewing surface to cause the display layer to display theoptical characteristic of the high negatively charged particles at theviewing surface; (c) once the optical characteristic of the highpositively charged particles is displayed at the viewing surface,applying across the electrophoretic medium a low electric field having apolarity driving the low negatively charged particles towards theviewing surface to cause the display layer to display the opticalcharacteristic of the low negatively charged particles at the viewingsurface; and (d) once the optical characteristic of the high negativelycharged particles is displayed at the viewing surface, applying acrossthe electrophoretic medium a low electric field having a polaritydriving the low positively charged particles towards the viewing surfaceto cause the display layer to display the optical characteristic of thelow positively charged particles at the viewing surface.
 12. A drivingmethod according to claim 11, wherein the low electric field is 5% to50% of the high electric field.
 13. A method of driving anelectrophoretic medium comprising a solvent or solvent mixture and highpositively charged particles, high negatively charged particles, lowpositively charged particles, and low negatively charged particles alldispersed in the solvent or solvent mixture, the four types of particleshaving respectively optical characteristics differing from one another,which method comprises, in any order: (a) applying across theelectrophoretic medium a high electric field of one polarity to causethe electrophoretic medium to display the optical characteristic of thehigh positively charged particles; (b) applying across theelectrophoretic medium a high electric field of the opposite polarity tocause the electrophoretic medium to display the optical characteristicof the high negatively charged particles; (c) once the opticalcharacteristic of the high positively charged particles is displayed,applying across the electrophoretic medium a low electric field of saidopposite polarity to cause the electrophoretic medium to display theoptical characteristic of the low negatively charged particles; and (d)once the optical characteristic of the high negatively charged particlesis displayed at the viewing surface, applying across the electrophoreticmedium a low electric field of said one polarity to cause theelectrophoretic medium to display the optical characteristic of the lowpositively charged particles.
 14. A driving method according to claim 13wherein at least one of steps (a) to (d) comprises application of ashaking waveform prior to application of the electric field.
 15. Adisplay device comprising two sub-pixels, each sub-pixel comprising anelectrophoretic medium according to claim 6 and comprising blackparticles and three types of non-black particles, the two sub-pixelscombined comprising particles of at least four different non-blackcolors.
 16. A display device according to claim 15 wherein the twosub-pixel combined comprise particles of at least six differentnon-black colors.